Atomic-force microscopy (AFM) is a technique for imaging a surface of a surface at the sub-nanometer (nm) scale. It has become commonly used for surface characterization, as well as mapping of certain material-specific surface properties, for materials such as polymers, ceramics, composites, glass, and biological tissue.
An AFM probe typically includes a very sharp tip that is mounted at the end of a cantilever. To image a surface, the tip is moved along the surface and its interaction with the surface is recorded. When the tip is in proximity with the surface, forces between the tip and the sample lead to a deflection of the cantilever, which can be measured with very high accuracy. By scanning the probe tip in two dimensions, a complete map of the surface structure and/or other physical properties of the surface can be developed.
Tapping-mode AFM (TM-AFM) is a particular type of AFM wherein the probe tip is brought into intermittent-contact with the surface so that it intermittently touches or “taps” the surface. TM-AFM is particularly attractive for measuring soft materials, since the tip is less likely to be “stuck” in the material. In addition, lateral forces on the tip, such as drag (which can reduce measurement accuracy), are virtually eliminated.
In TM-AFM, an actuator drives the cantilever such that it oscillates at its fundamental resonance frequency while the probe tip is scanned over the sample surface. The separation between the probe tip and the sample surface is adjusted via a feedback control loop to maintain constant oscillation amplitude at the probe tip. As the tip intermittently touches the sample during a scan, the tip experiences a contact force that induces dynamic effects on the mechanical behavior of the probe, such as oscillation amplitude changes, phase changes, and the develop of harmonic components. These effects occur at frequencies much higher than the fundamental frequency of the probe and contain information about the physical properties of the surface over which the probe tip is scanned. Unfortunately, conventional AFM probes lack the capability to measure the higher signal frequency components with sufficient fidelity.
To overcome the drawbacks of conventional AFM probes, AFM probes having higher bandwidth capability were developed to enable direct measurement (typically via optical means) of the higher frequency components of the tip-sample interactions while preserving conventional operation in tapping mode. Examples of high-bandwidth AFM probes are disclosed in U.S. Pat. No. 8,082,593, which is incorporated herein by reference.
A typical high-bandwidth AFM probe has a cantilever body that extends from a reference structure to a first end, from which a sensor cantilevers to a second end that includes the probe tip. The sensor is characterized by higher resonance frequency than the cantilever portion; therefore, the sensor can respond to the dynamic effects that arise from tip-surface interactions. The mechanical behavior of the sensor versus that of the cantilever body is monitored, providing a signal that can be processed to yield more information about the properties of the surface than can be obtained with simpler cantilever-type probes.
The performance of these high-bandwidth AFM probes is limited by their design, however. Sufficient measurement sensitivity and signal-to-noise ratio (SNR) can only be achieved when the probes are operated within a narrow range of operation conditions, such as a specific, narrow range of wavelengths and incidence angle for the optical signal used for their interrogation. Further, these sensors can only be operated in a medium having a refractive index that is within a very narrow range.
Improved AFM systems, probes, and methods that increase the range of operating conditions in which AFM microscopy can be performed would represent a significant advance in the state of the art.